This invention relates to a process and apparatus associated with the reduction of the sulfur oxide content of combustion gases produced by the combustion of organic fuels containing sulfur. In particular, the invention relates to the reduction of sulfur dioxide content under conditions which also reduce the fouling of heat exchanger tubes exposed to the combustion gases.
Combustion of sulfur bearing fuels forms sulfur dioxide and some sulfur trioxide which combine with moisture within the environment to produce what is commonly called acid rain. Such environmental intrusion is of great concern and has lead to federal regulations which establish emission standards. Several processes to control the emission of sulfur dioxide presently exist. However, in many instances these processes involve the use of expensive process equipment which adds to the cost of the operation.
One conventional gas desulfurization process involves the use of lime/limestone scrubbers to form CaSO.sub.3 /CaSO.sub.4 from the sulfur oxide. In this process, a lime slurry is sprayed into the combustion gas which reduces the temperature of the gas. Usually a reheating of the gas is required before the gas enters the stack. The cost of the scrubbers, their operation in the process and the reheating of the combustion gas add to the cost of the overall operation. Processes which illustrate the use of aqueous alkaline reactants for sulfur dioxide removal are disclosed in U.S. Pat. Nos. 4,197,278, 4,198,380, 4,410,500, 4,385,039, 4,325,713, 4,208,381.
In another process, aqueous sodium sulfite or a mixture with sodium sulfate is used as an absorption solution for contact with sulfur dioxide. U.S. Pat. Nos. 4,079,119 and 4,206,187 provide disclosures of these processes.
When the combustion process is part of glass-making operations, sodium hydroxide or other sodium compound is contacted with the combustion gas to form sodium sulfite or sodium sulfate. U.S. Pat. Nos. 3,944,650; 4,001,384; and 4,176,163.
A different process as described in U.S. Pat. No. 3,369,504 involves the addition of finely divided or gaseous NaCl to the boiler to react with sulfur dioxide and/or sulfur trioxide. Scrubbing may be used to remove the hydrogen chloride and sodium sulfate is removed with the boiler ash.
An additional problem is often associated with sodium sulfate when it is formed in or near the furnace. In conventional combustion processes, heat from the hot combustion gases is transferred to boiler tubes in the furnace and subsequently to a plurality of auxiliary heat exchangers downstream from the furnace. When Na.sub.2 SO.sub.4 is formed upstream from these heat exchangers at the higher temperatures associated with sulfate formation, the Na.sub.2 SO.sub.4 is usually liquid at the higher temperatures and tends to deposit on the heat exchanger surfaces together with ash particles thereby reducing the effectiveness of the heat transfer operation.
Accordingly, one object of the invention is a process and associated apparatus for the reduction in the sulfur oxide content of gas formed from combustion of sulfur-bearing fuels. A second object is the conversion of sulfur oxide to a sulfate product at higher temperatures associated with the furnace or adjacent downstream sections. A third object is a process for reducing harmful effects of Na.sub.2 SO.sub.4 in a combustion gas without substantial modification of existing equipment. Another object of the invention is the avoidance of adding heat to the flue gas prior to entering the stack. A further object is the reduction in heat exchanger fouling associated with some systems for reducing the sulfur oxide content. Yet another object of the invention is one or more combinations of the above objects. These and other objects will become apparent from the following detailed description.